Electric power systems typically employ turbine generators to supply real power to the system grid in order to meet load demands of the system, and to supply reactive power to the system for supporting system voltage, thus enabling the power system to successfully provide electrical power. A turbine generator typically comprises a synchronous generator with its excitation system and a turbine to drive the synchronous generator. The driving power from the turbine allows the generator to transmit real power to the system, and the excitation system allows the synchronous generator to provide needed reactive power and voltage support.
As a power plant ages, high maintenance costs, lack of efficiency, or regulatory requirements may dictate that the turbine and its associated auxiliaries, such as boiler and reactor, be retired. In such case, the turbine, although still having a useful life, is no longer available to drive the generator, and the generator without a turbine to drive it will no longer be capable of supplying real power to the system. In these cases, traditionally, the entire plant, including the generator, has been retired or decommissioned. Because of the changing electric utility industry and the need for reactive power at critical locations in the power system, many power plant owners of retired power plants or equipment within the plant have considered decoupling the turbine from the generator and using the generator with its excitation system to supply reactive power as a synchronous condenser.
A synchronous condenser is a synchronous machine that delivers and absorbs reactive power to and from a transmission grid of a power system, but does not deliver real power to the grid during operation because it is typically not being driven by a prime mover such as a turbine. In order to synchronize the synchronous condenser to the transmission grid, it must first be driven to a rated speed by a prime mover, such as a turbine, electric motor, or torque converter, or by a starting package. Once the synchronous condenser has been synchronized to the grid, the prime mover or starting package is typically disconnected or de-energized. The use of synchronous machines is well known in the art, as illustrated by way of example with reference to U.S. Pat. No. 3,772,526 to Alwers directed to an apparatus for starting a gas turbine which is shaft coupled to a synchronous generator and bringing the turbine up to speed where it is able to supply sufficient torque to further accelerate the generator up to its synchronous speed for connection into a power system. Various methods have been disclosed for controlling rotational speed and phase of synchronous motors such as described in U.S. Pat. No. 4,418,307 to Hoffman et al. which senses the motion of poles with respect to the stator for providing sensor signals, which signals are then converted to sensor pulses and compared to a constant pulse for altering braking and accelerating moments. Further, it is well known to use synchronize a power plant generator with a power system by simply bring operating equipment up to a desired or rated speed, as described with reference to U.S. Pat. No. 4,031,407 to Reed.
However, a problem associated with driving the synchronous condenser to a rated speed and synchronizing the condenser to the grid includes controlling the prime mover or starting package. Typically, controllability of a prime mover, such as a torque converter, is quite coarse, while requirements for synchronization to the grid may be quite fine and require precision performance. By way of example, if one is attempting to synchronize an 1800 rpm synchronous machine to a grid in order to operate it as a synchronous condenser, its speed must typically be within .+-.1 rpm of the rated 1800 rpm in order to achieve a successful synchronization. However, the starting package which is used to drive the synchronous machine to the rated speed and then attempt to hold the machine near that rated speed in order to connect to the grid may nor be capable of holding the machine speed at the rated speed (e.g. 1800 rpm) or even within an acceptable range (e.g. .+-.1 rpm or .+-.2 rpm). As a result, synchronization may not be achievable by waiting until the speed of the starting package levels off to some steady state speed, because that steady state speed may be too far away from the rated speed to have a successful synchronization. By way of further example, if a starting package speed controller attempts to compensate for this error in speed by adjusting output torque of the prime mover, the change may be such that the speed stabilizes at some arbitrary higher or lower value to the rated speed, but still too far away from the rated, desirable, speed for synchronization. Continued attempts add costly operating expenses including valuable time for synchronizing. Further, any change in torque will typically take time to affect the speed due to a typically large rotor inertia being driven. The monitoring of rotor speed, as is typical in the art, involves a long delay time for any change in the starting package. Therefore, monitoring and controlling the speed will require a long time for any change in the starting package to affect a change in speed. It is desirable that the synchronization conditions be satisfied accurately and reliably, even with prime movers such as torque converters which typically have a coarse level of speed control. The present invention seeks to solve this unacceptable, yet to date tolerated problem.